Meissner effect in Y-Ba-Cu-O and La-Sr-Cu-O high-temperature superconductors.

Abstract

Measurements of the static magnetic susceptibility confirm the existence of significant Meissner flux expulsion in the high-temperature superconductors ${\mathrm{Y}}_{1.2}$${\mathrm{Ba}}_{0.8}$${\mathrm{CuO}}_{4}$ and ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ below the onset temperatures of 85 and 39 K, respectively. The Y-Ba-Cu-O sample contains both superconducting and nonsuperconducting phases. The observed Meissner effect corresponds at 20 G to at least 20% of perfect diamagnetism in the black superconducting phase. For the single-phase La-Sr-Cu-O compound the Meissner signal at 10 G reaches a value corresponding to at least 24% of perfect diamagnetism.